1. Field of the Invention
The present invention relates generally to an amplitude calculation circuit. More particularly, the invention relates to an amplitude calculation circuit for accurately calculating an amplitude in a base band (I signal and Q signal) of a communication device employing a quadrature phase modulation.
2. Description of the Related Art
In the conventional amplitude calculation circuit, an amplitude of a base band [I (In-phase) signal and Q (Quadrature-phase) signal] of a communication device employing an quadrature phase modulation is accurately calculated. In this circuit, from the I, Q base band signals, an amplitude expressed by:
A(t)=|G(t)|={square root over ( )}[I(t)2+Q(t)2]xe2x80x83xe2x80x83(1)
can be accurately derived.
wherein A(t) is an amplitude of an quadrature modulation wave, t is a time, G(t) is an quadrature modulation wave, I is an amplitude of a component (in-phase component) in-phase relationship with a carrier, and Q is an amplitude of a component (quadrature phase component) in an quadrature phase relationship with the carrier. The foregoing technology will be an important technology toward future in an quadrature phase modulation type communication system.
For example, it becomes necessary to derive an amplitude from received I and Q signals, to compare with a predetermined value, and to perform AGC (Automatic Gain Control). Conventionally, there is no method for deriving the amplitude from the I and Q signals simply and in high precision. In this circumstance, currently, an approximated value expressed by:
Axe2x80x2=max (|I|,|Q|)+xc2xdxc2x7min(|I|,|Q|)xe2x80x83xe2x80x83(2)
has been used. This contains significant error in comparison with the correct amplitude.
On the other hand, if a instantaneous amplitude can be accurately calculated from the base band signal at a transmission side, it becomes possible to perform control for increasing a bias current of a transmission power amplifier when the amplitude is large and for decreasing the bias current of the transmission power amplifier, using the result of calculation of the instantaneous amplitude. By performing such control, a distortion at a peak of the amplitude can be reduced with maintaining an average consumption of current.
Furthermore, currently, in order to achieving improvement of efficiency of a transmission amplifier, a predistorter which is a kind of linearizer is considered as promising. In the predistorter, accurate amplitude calculation is inherently required. An example of this is shown in FIG. 11.
In FIG. 11, an input signal Sr is consisted of a base band signal Ir of an in-phase component with a transmission carrier and a base band signal Qr of quadrangular-phase component of the transmission carrier. The input signal can be considered as a complex number taking the signal Ir as a real number portion and the signal Qr as an imaginary number portion.
The input signal Sr, namely the signal Ir as the real number portion and the signal Qr as the imaginary number portion operated by complex multiplication with a distortion correction data (real number portion is Re and imaginary number portion Im) from a ROM (read-only-memory) by a complex multiplier 20. The complex multiplier 20 comprises multipliers 1 to 4 and adder and subtracter 5 and 6.
The output of the complex multiplier 20 is a complex signal Sp, in which amplitude and phase of the input signal Sr are corrected so that a characteristics of a non-linear amplifier 11 becomes linear. As a result of complex multiplication, the complex signal Sp is expressed by:
Sp=Srxc2x7axc2x7exp(jxcex8)xe2x80x83xe2x80x83(3)
wherein a is an amplitude correction value and xcex8 is a phase correction value.
Accordingly, correction data are expressed by:
Re=axc2x7cos(xcex8)
xe2x80x83Im=axc2x7sin(xcex8)xe2x80x83xe2x80x83(4)
The complex signal Sp is a signal derived by multiplying the amplitude of the input signal Sr by a and phase thereof is rotated for xcex8 and can be calculated by using the real number portion Re and the imaginary number portion Im.
Assuming the real number portion of the complex signal Sp is Ip and the imaginary number portion is Qp, Ip and Qp are expressed by:
Ip=Rexc2x7Irxe2x88x92Imxc2x7Qr
Qp=Rexc2x7Qr+Imxc2x7Irxe2x80x83xe2x80x83(5)
The signals of the real number portion Ip and the imaginary number portion Qp are converted into analog signals by D/A (digital-to-analog) converters (DACs) 7 and 8 and then are converted into high frequency signals by a quadrature modulator 9.
On the other hand, an amplitude calculation circuit 15 calculates and outputs an instantaneous amplitude |Sr| of the input signal Sr. The instantaneous amplitude |Sr| is expressed by:
|Sr|={square root over ( )}[Ir2+Qr2]xe2x80x83xe2x80x83(6)
This equation (6) is the same as the equation (1).
On the other hand, the output of the non-linear amplifier 11 is branched by a coupler 12 and is rectified by a rectifier 19. Then, an average transmission amplitude is derived by a low-pass filter (LPF) 18. This signal converts into a digital signal by an A/D (analog-to-digital) converter (ADC) 17 to derive an average transmission amplitude.
The instantaneous amplitude |Sr| and the average transmission amplitude of the input signal Sr are multiplied by a multiplier 30. The result (product) of multiplication represents an instantaneous amplitude. This value is used as an address input for a distortion compensation ROM (look-up table) 14.
In the conventional amplitude calculation circuit set forth above, it is required to calculate quite accurately. In order to realize this, a method to read out the amplitude from ROM table with taking the I and Q signals as addresses.
This method has been disclosed in xe2x80x9cQuantization Analysis and Design of a Digital Predistortion Linearizer for RF Power Amplifiersxe2x80x9d (Sundstrom. L.; Faulkner, M.; Johansson, M., Vehicular Technology, IEEE Trans., Vol. 45 4, page 707-719).
However, in such method, ROM having quite large capacity becomes necessary for deriving accurate amplitude. This has been the most important problem. As set forth above, it has been important task for calculating accurate amplitude from the I, Q base band signals irrespective of transmission side or reception side in the quadrature phase modulation type communication device.
The present invention has been worked out in view of the problems set forth above. Therefore, it is an object of the present invention to provide an amplitude calculation circuit which can calculate an accurate amplitude with quite small circuit scale and quite small power consumption.
According to the first aspect of the present invention, an amplitude calculation circuit comprises:
a plurality of circuits, each including
an absolute value calculating circuit receiving a pair of base band signals and calculating respective absolute values thereof; and
a phase rotation circuit receiving the absolute values as components of two-dimensional vector and rotating the two-dimensional vector over a predetermined rotational angle for outputting as component of the vector;
the plurality of circuits being connected in cascade connection for receiving respective of the base band signals as input signal at a first stage and outputting an output of the phase rotation circuit of a final stage as a result of amplitude calculation.
Namely, the amplitude calculation circuit of the present invention relates to the circuit for calculating the amplitude of a high frequency signal from a values of I and Q base band signals in a radio transmitter device generating a high frequency signal through an quadrature phase modulation of I and Q base band signals or in a radio receiver device reproducing the I and Q base band signals by quadrature demodulation of a received high frequency signal.
The amplitude calculation circuit according to the present invention is a digital signal processing circuit. The I (in-phase component) signal and the Q (quadrature-phase component) signal as the input signals are also digital base band signals. These digital base band signals are digital signals before D/A (digital-to-analog) conversion in the radio transmitter device and digital signals after A/D (analog-to-digital) conversion of an analog base band signal received in the radio receiver device.
In the amplitude calculation circuit according to the present invention, the base band signals of quadrature phase modulation wave are two kinds of signals of I signal and Q signal. The quadrature phase modulation wave G(t) is expressed as follow by using I and Q signals:
G(t)=I(t)xc2x7cos(2xcfx80fcxc2x7t)xe2x88x92Q(t)xc2x7sin(2xcfx80fcxc2x7t)xe2x80x83xe2x80x83(7)
wherein t is a time, and fc is a frequency of carrier.
When carrier F(t) is expressed by:
F(t)=pxc2x7cos(2xcfx80fcxc2x7t)xe2x80x83xe2x80x83(8)
I is an amplitude of a component (namely in-phase component) of in-phase relationship with the carrier, and Q is an amplitude of a component (namely quadrature-phase component) of quadrature phase relationship with the carrier.
From the foregoing equation (6), the amplitude A(t) of the quadrature phase modulation wave becomes the foregoing equation (1). In practice, the amplitude A(t) becomes a value of constant multiple of the value derived by the equation (1) with a gain of a radio circuit, A/D converter, D/A converter or the like. However, it can be defined that the amplitude A(t) is calculated through the equation (1) without losing general applicability.
The present invention is directed to a method for configuring a circuit for simply deriving a value proportional to the amplitude A(t) of the quadrature phase modulation wave from the I signal and the Q signal. In order to accomplish this, the amplitude calculation circuit according to the present invention, an amplitude calculation circuit comprising:
a plurality of circuits, each including
an absolute value calculating circuit expressed as Ak, receiving a pair of base band signals Xk and Yk, where k is in a range of 0 to N which is a positive integer, and calculating respective absolute values |Xk| and |Yk| thereof; and
a phase rotation circuit expressed as Rk, receiving the absolute values |Xk| and |Yk| as components Xin,k and Yin,k of two-dimensional vector Vin,k and rotating the two-dimensional vector over a predetermined rotational angle xcex8 for outputting as components Xout,k, Yout,k of the vector Vout,k;
the circuits of k=0 to N being connected in cascade connection, input signals X0, Y0 of the first stage being input as respective base band signals I and Q and outputs an output Xout,N of the phase rotation circuit RN at the final stage as a result of amplitude calculation.
In the amplitude calculation circuit, assuming that input signals of the phase rotation circuit Rk are Xin,k and Yin,k and output signals are Xout,k and Yout,k, the phase rotation circuit comprises a first shift circuit shifting the input signal Xin,k for k bits, a second shift circuit shifting the input signal Yin,k for k bits, an adder for adding the input signal Xin,k and a result of shifting operation of the second shift circuit, and a first subtracter for subtracting a result of shifting operation of the first shift circuit from the input signal Yin,k, an output of the adder is taken as the output signal Xout,k and an output of the first subtracter is taken as the output signal Yout,k.
Also, in the amplitude calculation circuit, for the phase rotation circuits Rk, in which k is greater than 1, the signal Xk is directly input as the input signal Xin,k bypassing the absolute value circuit Ak, and an absolute value |Yk| of the signal Yk derived via the absolute value circuit Ak is input as the input signal Yin,k.
Furthermore, in the amplitude calculation circuit, the phase rotation circuit Rk, in which k is within a range from 0 to Nxe2x88x921, further comprises a second subtracter for subtracting the input signal Yin,k from a result of shifting operation by the first shift circuit, an output of the second subtracter is taken as xe2x88x92Yout,k,
the absolute value circuit Ak+1 at the stage next to a stage where the phase rotation circuit Rk is present, performs calculation of the absolute value by selective outputting Yout,k when a value of the output signal Yout,k is positive and xe2x88x92Yout,k when the value of the output signal Yout,k is negative.
The phase rotation circuit RN at the final stage may exclude the first subtracter, the second subtracter and the first shift circuit and thus output only the output signal Xout,N without outputting the output signal Yout,N and xe2x88x92Yout,N.
The amplitude calculation circuit may insert delay means in a signal transmission path between each phase rotation circuit Rk and each absolute value circuit.
By this, it becomes possible to obtain a quite accurately calculated value of the amplitude with a combination of lesser number of the absolute value circuits, the shift circuits, the adders and the subtracters. In this case, no multiplier which requires high power consumption and large circuit scale, is used. Accordingly, in comparison with the amplitude calculation circuit with a ROM table used in the conventional predistributor, calculation of the amplitude with higher precision can be done with smaller circuit scale and lower power consumption.